Print this pagePre-, Syn-, and Post-Rift Sedimentation in the Middle to Late Jurassic and Earliest Cretaceous in the Southern Nordland Ridge Area, Mid-Norway
RORNES, ARNT E., EDWARD H. CLIFTON, PAUL MILNER, TORUNN SVINDLAND, and STEVE THOMAS
A sequence stratigraphic analysis based on seismic, biostratigraphic, and depositional facies data indicates a complex interplay of eustasy and tectonism as Middle to Late Jurassic and Early Cretaceous sediment accumulated during episodes of rifting in the Southern Nordland Ridge Area, Mid-Norway.
During much of the Middle Jurassic (Aalenian/Bajocian), paralic and fluvial sediment accumulated in a setting largely unaffected by faulting. The section expands without change in facies across the major fault separating the Nordland Ridge from the Donn Terrace, indicating offset too slow to generate physiographic relief. In the late Bajocian-early Bathonian, a regional rise in relative sea level drowned the coastal/alluvial plain and led to deposition of marine shale and prograding parasequences. Shortly thereafter, accelerated rifting broke the area into a complex of horsts and grabens. Reduced sedimentation on the horst blocks led to condensed sections punctuated by 2 erosional hiatuses (early Bathonian-mid Callovian and early Oxfordian-late Oxfordian) during probable lowstands. Sediment released during these erosional intervals accumulated as turbidites or related deposits in the well-developed grabens and bypassed through those less well-developed. Expansion of the section in these latter basins appears to be confined to late Oxfordian-early Ryazanian deposits. A drape of highly condensed marly shale, conformably overlying organic-rich mudstone of uppermost Jurassic-lowermost Cretaceous age, reflects probable maximum flooding at the close of the late Jurassic rifting. Younger Early Cretaceous strata are preserved only in areas of pronounced subsidence in subsequent episodes of rifting. Elsewhere, these deposits have been eroded during the low stands reflected by unconformities at the base of the Aptian, the top of the Aptian, and the base of the Albian, and intra-Turonian. Following the erosional episode in the intra-Turonian, sea level rose and marine shale of the Shetland Formation accumulated over the area.